Why are flags rectangular?

not all of them are. Some countries have triangular flags…

If this shape has a name, I don’t know what it is.

Edit. I deleted myself. I apologize if anyone read my original answer.

I stand corrected: http://en.wikipedia.org/w/index.php?title=National_flag#Unusual_flags One is triangular. 2 are square.

Squares are rectangular.

Interestingly, the state flag of Ohio is the only non-rectangular shaped state flag in the USA.

So they can flap in a breeze really well.

@loser I thought of posting that, but it seemed to easy :)

Its a custom I guess. prolly dating why back to banners.